Monoclonal antibodies specific for the unaggregated form of arginyl-tRNA synthetase which we have purified from human placenta will be isolated. The antibodies will be isolated from supernatants of growing hybrid cell cultures derived by fusion of mouse myeloma cells (P3/X63 Ag8 or SP2/0 Ag14) with spleen/cells derived from Balb/C mice previously immunized with the human enzyme. Antibodies against the human enzyme will be assayed by inhibition or binding of the human enzyme. The antibodies obtained will be screened for species-specific reaction with human arginyl-tRNA synthetase. Antibodies which bind the human but not the mouse enzyme will be used to assay the presence of human arginyl-tRNA synthetase in mouse-human somatic cell hybrids in order to map the gene for the human enzyme. Antibodies against the unaggregated form of human arginyl-tRNA synthetase will be tested for reaction with the aggregated form of the enzyme from placenta. Antibodies which cross-react will be employed to detect the physical associations between different aminoacyl-tRNA synthetase activities in aggregated preparations of these enzymes. Using the procedures developed for arginyl-tRNA synthetase, we will begin to isolate monoclonal antibodies specific for other aminoacyl-tRNA synthetases purified from human placenta. These antibodies will be useful both in mapping and biochemical studies of the enzymes.